In the usual fork-type lift truck, the forks are raised by a vertically extensible lift mast arrangement located at the forward end of the truck. The forks are placed beneath a load, are initially raised a limited amount for transportation purposes, and are thereafter raised to a desired elevation for depositing the load on a stack, shelf or the like. Such a mast arrangement usually includes a rearwardly disposed support assembly having at least one pair of upright channular beams, a forwardly disposed carriage assembly mounted by a plurality of rollers for rolling engagement longitudinally within the beams, and a motor or lifting device to cause relative vertical dispacement therebetween. Exemplifying the prior art in this area are the following U.S. Pat. Nos.:
2,759,562 issued to B. I. Ulinski on Aug. 21, 1956. PA1 3,213,967 issued to R. Hastings, Jr et al. on Oct. 26, 1965. PA1 3,556,247 issued to A. Shinoda et al. on Jan. 19, 1971. PA1 3,768,595 issued to W. H. Kelley, Jr. on Oct. 30, 1973.
For the most part, the aforementioned lift mast constructions have experienced distortion of the channular beams due to the loading of the rollers under typically adverse circumstances and severe flaking of the beams by the rollers after extended service due to high contact stresses. A considerable portion of these problems is caused by offset loads on the forks which tends to cock or rotate the carriage assembly relative to the channular beams which guides it. To solve this, many arrangements of rollers have been adopted, including the addition of side thrust rollers which serve to transmit lateral forces and to relieve the longitudinally oriented rollers of this function. Upper and lower pairs of guide rollers, as well as another intermediately elevational spaced pair of guide rollers are being used with separate side thrust rollers disposed in various locations in the limited space often found in these lift masts.
More recently, canted rollers have been commercially introduced on lift mast carriage arrangements whereby the upper rollers are canted to contact the inside corner angles adjacent the front rail surfaces and webs of the channular beams and the lower rollers are oppositely canted to contact the corner angles adjacent the rear surfaces of the channular beams. In this way it is possible to omit one or two pairs of side thrust rollers in certain lift masts. However, the roller engaging surfaces of the beams then experience a reduced wear life because the canted guide rollers do not transmit direct in-line forces. Moreover, the service life of the canted guide rollers is reduced over straight guide rollers because more side thrust is absorbed thereby.
It is to be appreciated that it is difficult to accurately and positively adjust the position of the usual longitudinally oriented rollers and the position of the side thrust rollers in order to distribute the loads evenly and to stabilize the carriage assembly as it is positioned in the channular beams. Moreover, this adjustment problem becomes more complex when canted guide rollers are used, and the accurate shimming and manufacturing thereof is considerably more expensive to undertake.
Still another major factor relates to the advantages associated with a six roller carriage assembly, wherein the carriage assembly can advantageously be elevated beyond that of a four roller assembly because the upper pair of rollers are allowed to travel upwardly beyond the extremities of the channular beams. This complicates the roller arrangement at the upper portion of the carriage assembly since both longitudinal and transverse guiding must still be retained.
Another factor involves the tendency of certain side thrust roller arrangements to laterally spread the beams, so that it is further considered to be desirable to transmit offset loads on the carriage assembly solely into one of the beams.